Pipe under joists

[Ian Clowes]

Hi

I think it was Comets that suffered from this particularly, messing up what was otherwise seen as a pretty sucessful aircraft.

Although it discusses ships and hatch shapes rather than airplanes the book 'The New Science of Strong Materials: Or Why You Don't Fall Through the Floor' by J.E. Gordon covers this behaviour, called Griffith cracking, IIRC. The 'magic' of these things is that once you reach a critical crack size catastrophic failure is inevitable.

One story in there is about a ship that had a crack developing. One of the galley crew (because this is where the crack was, IIRC) took to marking the progression of the crack by dating the tip of the crack periodically.

Eventually the ship split and sank. Luckily (for science) the recovered the half of the ship with the marks on it. These marks become the first record of a Griffith crack 'in the field' and showed that the ship had carried on sailing for years whilst the crack was below the critical size, but once it reached the theoretical failure point the next heavy seas took her down.

I may have elements of that wrong, but I think the gist is right. The book quite a good read for other material stuff you've probably always wondered about too.

IanC

[PoP]

I think I'll go with gut feel :)

PoP

[Tony Bryer]

With Liberty ships the key problem was the previously unknown problem of brittle fracture: normal steel becomes brittle about freezing point and then if overstressed will fracture rather than yield.

Comet problem was to do with metal fatigue, with the square corners windows causing concentration of stresses.

To get back on topic, good practice for notching joists is to drill and cut, though of course it depends how much one is taking out the joist.

[jerrybuilt]

with the Liberty ships was design/welded construction. The problem with the Titanic was sloppy procedure.

Try an experiment. Get two nails, and two pairs of pliers. Take a nail at room temperature in the pliers, and bend it. Put the other nail and the pliers in the freezer for 1/2 an hour, then put on gloves, pick up the pliers, and bend the nail. It won't snap. --

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[The Natural Philosopher]

Gut feel is OK if like many many craftsmen, you have been using the materials for years and have experienced failures in them.

Otherwise, stick to someone esses rules, or do the (very complex) calculations, or take the engineering route, and put the structures in a machine and stress them till they break.

:-)

[The Natural Philosopher]

It fatigues much quicker tho.

[PoP]

I should expand on that previous statement a bit.....

By "gut feel" I think I'd be swayed towards "I ain't gonna drill that size hole in that size joist!", even if it looked as though it ought to be okay.

Some things aren't worth taking chances with.

PoP

[Dave D]

Cheers All... So basically I can drill up to a 25mm hole in a 100mm joist. But not necessarily the 35 mm I require for a kitchen waste output. Is there a regulation minimum for a kitchen waste pipe, thanx again. Will this run and run ??? :-)

[Dave Plowman]

I wonder if how you fix the floorboard above a notched joist makes a difference? I've a gut feeling it does...

[Pablo]

According to my Onsite Guide (for Electricians) the maximum hole diameter should be 0.25 * joist depth. The maximum depth of a notch should be 0.125 * joist depth.

Holes should b on a centre line in a zone between 0.25 and 0.4 * span of joist. (This requirement is usually ignored) Notches on top in a zone between 0.1 and 0.25 * span of joist.(This requirement is usually ignored)

[Owain]

"Dave D" wrote | Cheers All... | So basically I can drill up to a 25mm hole in a 100mm joist. | But not necessarily the 35 mm I require for a kitchen waste output. | Is there a regulation minimum for a kitchen waste pipe, thanx again. | Will this run and run ??? | :-)

If you get the fall on the pipe wrong it won't run at all :-)

Owain

[Mr Fiendish]

Not if it gets blocked! (sorry, couldn't resist!)

Barley Twist (Please put out the cats to reply direct)

[Ed Sirett]

IIRc the hole must not be more than 20% of the joist depth which means that the joist must be at least a 7x2. The hole edges should be at least

50mm from the bottom and 50mm from the top of the joist. Also the hole position should be between 10% and 20 (or 80-90% at the other end) of the span length .

Boring the hole between joists that are square to the joist requires some seriously good tools which I'm going to have to buy for a current job. I need to get a 22mm gas pipe through - the pipe will be fed in from the outside.

[PoP]

I've been looking at those side-on drilling tools as well. There don't seem to be any bargain-basement versions unfortunately.

Bit of a pity really because if there was a Ferm or Nutool version of such a beast then I imagine it might do reasonably well in terms of sales. It's the sort of tool you need in the back of the wagon for the odd occasion when you need it, but the cost most likely doesn't lend itself to acquiring one for most people.

PoP

[BillP]

HI Dave,

Sorry to pee on your parade but if it's a kitchen sink waste I believe you'll find it's 40mm not 32mm

[The Natural Philosopher]

yes.

[John]

35mm means a big hole or notch out of the joist. a lot depends on the position of this notch relatie to the size of the joist and its position away from the support which affects the forces acting upon the remaining wood.

[BillP]

I'm still curious as to how Dave gets his 40mm waste through a 35mm hole.

[impvan]

Not quite. 30% of the strength when unloaded maybe. But when the joist is in place and loaded then the stresses are carried in the top and bottom of the joist leaving the centre zone with little load. So you can make bigger holes - and that's why notches in the top and bottom have to be much smaller.

[Grimly Curmudgeon]

bought the one illustrated here some two years ago and its proved its worth many times over. There is a cheaper version, but the price isn't listed on that page, but afair, it was around the 25 quid mark.